(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) unit and a LCD system and, more particularly, to LCD unit and system including a stacked LCD devices. The present invention also relates to a drive circuit for driving such a LCD unit or LCD system.
(b) Description of the Related Art
LCD units have the advantages of lower power dissipation and higher definition, and thus are used from a portable cellular phone to a large-screen monitor TV. The contrast ratio of a LCD device or LCD panel alone in the LCD unit is at most around 1000:1 in a dark environment, and thus is inferior to the contrast ratio of a CRT (cathode ray tube) or discharge-type display panel, such as PDP (plasma display panel), FED (field emission display) and SED (surface-emission electron-emitter display). For example, the PDP, which is generally used as a monitor TV similarly to the LCCD unit, has a contrast ratio of 3000:1. Thus, the LCD unit has a problem in that when a video source, such as movie, having an abundant power of expression in a dark portion, is used for display of the image on the LCD unit, there is insufficient sense of presence on site.
To solve the above problem, a technique is proposed which controls the intensity of the backlight for the LCD unit based on the picture image to be displayed, without improving the contrast ratio of the LCD unit itself, to improve the contrast ratio of the LCD unit as a whole. However, in the LCD unit having a surface-emission light source, a cold cathode tube having a narrower dynamic range of luminance is generally used as the backlight source. This narrower dynamic range limits the contrast ratio of the LCD unit in the range of 2000:1 to 3000:1 at most even if the light intensity of the backlight unit is controlled based on the picture image to be displayed. In addition, since the cold cathode tube is of a rod or cylindrical shape, the light intensity cannot be controlled if the image includes a higher luminance portion and a lower luminance portion at the same time on the same screen. This limits the improvement of the contrast ratio by the luminance control of the backlight. More specifically, if a picture image having both higher and lower luminance portions is controlled particularly in consideration of reproducibility for the lower luminance portion, the effective contrast ratio is lowered.
In order not to incur the above problem, it is generally necessary to intensively raise the contrast ratio of the LCD panel itself in the LCD unit. However, as described before, the contrast ratio of the LCD panel itself is at most around 1000:1 even if the contrast ratio of the LCD panel itself is improved. Patent Publication Nos. JP-1989-10223A and JP-1984-189625A describe a technique for considerably improving the contrast ratio of the LCD unit without significantly improving the contrast ratio of the LCD panel itself. In this technique, a plurality of LCD panels or LCD devices are stacked one on another in a LCD unit, to thereby reduce the dark luminance and thus raise the overall contrast ratio of the LCD unit.
FIG. 12 shows the configuration of a LCD unit including two LCD panels (LCD devices) stacked one on another. The LCD unit includes, as viewed from the light-incident side, polarizing film 901, LCD panel 941, polarizing film 902, LCD panel 942, and polarizing film 903, The LCD panel 941 includes a twisted-nematic mode (TN-mode) liquid crystal (LC) layer 931, and a pair of transparent substrates 911 and 912 each having transparent electrode or electrodes 921, 922 on the surface of the transparent substrate near the LCD layer 931. The LCD panel 942 includes a TN-mode LC layer 932, and a pair of transparent substrates 913 and 914 each having transparent electrode or electrodes 923, 924 on the surface of the transparent substrate near the LC layer 932. The transparent electrodes 921 and 923 are pixel electrodes to which a drive signal is supplied from a drive circuit 951, whereas the transparent electrodes 922 and 924 are common electrodes This configuration of the LCD unit provides an improvement of the contrast ratio from around 10:1 or 15:1 up to around 100:1. A LCD unit including three LCD panels having a similar structure may have a contrast ratio of around 1000:1. In short, the LCD unit having a plurality of LCD panels has a contrast ratio which exceeds the limit of the contrast ratio achieved by a single LCD panel.
In the LCD unit described in JP-1989-10223A, the higher contrast ratio is achieved by driving two LCD panels 941 and 942 by using the same drive signal supplied from a single video source. In this configuration, the distance between the LCD panel 931 and the LCD panel 932 as viewed in the thickness direction thereof provides a deviation of the location therebetween, when the display unit is observed in a slanted direction slanted from the perpendicular of the LCD panels. The deviation of location incurs a sense of discomfort to an observer observing the LCD unit in the slanted direction, due to the abnormal image or double-line image. In addition, there may be a case wherein light passes through both the LCD panels at different positions or at different color filters in the slanted direction, to thereby reduce the luminance and thus degrade the visibility of image by the observer.